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A TIP-OFF FOR BETTER QUALITY SCANNING PROBE MICROSCOPY Using scanning probe microscopy (SPM) to visualize and to manipulate individual atoms requires an atomically sharp metal tip, yet these actions compromise tip quality. This reduction in sharpness can cause a loss of atomic resolution, leading to “double tips,” or the appearance of secondary atomic features. Treatments such as short voltage pulses between the tip and sample or controllably indenting the tip into the sample can return the tip to atomic sharpness; however, these fixes can be time-consuming and present a roadblock toward the goal of fast, precise, and autonomous SPM operation. Rashidi and Wolkow (DOI: 10.1021/acsnano.8b02208) offer a solution, reporting a method to automate tip conditioning with machine learning. Using a hydrogen-terminated silicon surface as a model system, the researchers tested various machine-learning methods to recognize dangling surface bonds. They used approximately 3500 scanning tunneling microscopy images of isolated dangling bonds selected from their own archived data for training these systems. Of all the methods tested, a convolutional neural network yielded the best accuracy, achieving identification of degraded tips in 97% of test cases. The authors were able to improve this accuracy to 99% by using multiple points of comparison and majority voting. In addition, they were able to automate their system further by incorporating in situ tip conditioning upon detection of degraded probe quality. They show the viability of this framework through the fabrication of a binary atomic wire from silicon dangling bonds. The authors suggest that this advance is an important step toward creating autonomous atom-scale fabrication tools.
accuracy for transdermal detection and the bulky and expensive instrumentation required to measure signal output. In a step toward overcoming these challenges, Sun et al. (DOI: 10.1021/acsnano.8b02188) report an ultrasensitive optical transducer that can be used for wireless glucose monitoring through a smartphone. Their system relies on semiconductor polymer dots linked to an oxygen-consuming enzyme. After investigating several oxygen responsive dyes, the researchers settled on a palladium porphyrin complex. Compared to platinum complexes, the top-performing palladium complex exhibits significantly higher sensitivity in both in vitro and in vivo glucose detection. Using this ultrasensitive transducer, the researchers were able to obtain optical readings of subcutaneous glucose using the camera on a smartphone. To analyze these results, they developed an imageprocessing algorithm to decompose the luminescence image using an RGB model and a smartphone app to interpret these results. The researchers suggest that these advances could lead to an innovative optical monitoring platform for diabetic healthcare.
THE EYES HAVE IT Ocular diseases are relatively common and drive up healthcare costs with procedures including visual acuity tests, prescriptions of eye drops, corrective lenses, and eye surgeries. However, there is a lack of quantitative, point-of-care diagnostics to aid ophthalmologists. One potential tool to develop into a diagnostic is contact lenses, the ubiquitous devices worn to correct common vision problems. Integrating nanoscale features into contact lenses could offer a way to assess early symptoms of problems including dry eye, Graves’ ophthalmopathy, or glaucoma. However, methods used thus far to fabricate nanostructures on contact lenses suffer from a bevy of drawbacks, including multiple steps, high-energy supplies, long fabrication times, complex setups, special equipment, and damage to the lenses’ soft surfaces. AlQattan et al. (DOI: 10.1021/acsnano.8b00222) report a technique of inscribing contact lenses with nanoscale features using a holographic laser ablation method. The researchers
A BRIGHT IDEA TO MONITOR GLUCOSE Tight glycemic control is key to reducing diabetes complications. Over the past few decades, several monitoring technologies have been developed to detect glucose levels in blood or interstitial fluid. The most common method for continuous monitoring uses a needle-like, enzyme-coated electrochemical sensor implanted subcutaneously. However, this type of sensor has several disadvantages, including in vivo sensor degradation, poor response at low glucose concentrations, pain of insertion, and risk of infection. Although optical methods can counteract these drawbacks, thus far, the clinical application of optical glucose sensors has been severely limited by their inability to meet the required sensitivity and © 2018 American Chemical Society
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Published: June 26, 2018 5069
DOI: 10.1021/acsnano.8b04412 ACS Nano 2018, 12, 5069−5072
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Cite This: ACS Nano 2018, 12, 5069−5072
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In Nano
relied on a neodymium-doped yttrium aluminum garnet laser in holographic interference patterning mode to produce onedimensional and two-dimensional nanostructures over the surfaces of hydrogel contact lenses. To avoid interfering with vision, these features were placed on the lens edges. Based on changes in optical diffraction, which depends on electrolyte concentrations, the researchers were able to use these laserinscribed lenses to diagnose simulated dry eye. The patterning could also be used to diagnose conditions that change pressure in the eye, such as glaucoma, which affects the curvature of the lens and its nanofeatures. The authors suggest that these lenses could be further functionalized to diagnose other ocular conditions through concentration changes of analytes and proteins in tears.
HELPING CANCER VACCINES REACH THEIR TARGET Methods that harness a patient’s own immune system have become a growing part of the anticancer arsenal. Of these, cancer vaccines have relatively low costs, high specificity to attack tumor cells, and low side effects. Thus far, the antigens used to trigger specific antitumor immune responses have included tumor-specific peptides, proteins, DNA, mRNA, and whole tumor cell lysates. However, vaccines that incorporate these antigens have shown varied efficacies in different cancer patients. Although wrapping nanoparticles with membranes of different cell types has been used as a strategy to craft other types of vaccines, few reports exist on this approach for cancer vaccines. Yang et al. (DOI: 10.1021/acsnano.7b09041) tested this strategy, constructing a nanovaccine formulation by coating adjuvant nanoparticles with tumor cell membranes. The researchers loaded poly(D,L-lactide-co-glycolide) nanoparticles with R837, an agonist against toll-like receptor 7, and then encapsulated them with membranes from B16-OVA cancer cells. They further modified these nanostructures with mannose through a lipid-anchoring method. Because mannose binds to specific receptors on antigen-presenting cells such as dendritic cells (DCs), this modification could enhance DC uptake and lead to a stronger trigger for DC maturation. Tests showed that after intradermal injection in an animal model, these nanoparticles effectively migrated to draining lymph nodes and triggered tumor-specific immune responses. This vaccine inhibited tumor growth in animals challenged with tumors. In addition, when combined with a checkpoint blockade, some animals became tumor-free. The authors suggest that this approach shows significant clinic translational potential, with promising results for efficacy and all biocompatible materials.
GETTING IN TOUCH WITH MACHINES With burgeoning interest in wearable electronics, electronic textiles (E-textiles) have also been growing in demand. The design challenges of these materials are significant, including air permeability, flexibility and washability, pattern diversity of the electrode, and scalable production. In addition, for E-textiles to be light and convenient, they will require power sources that iare different from heavy, traditional batteries. One option for power are triboelectric nanogenerators (TENGs), which generate electrical charges on the basis of the triboelectrification effects and electrostatic induction. Bringing these concepts together, Cao et al. (DOI: 10.1021/ acsnano.8b02477) developed a washable, breathable E-textile that is powered by a TENG. The researchers created this material by stacking three layers: a top layer of silk, serving as one frictional material; a bottom layer of nylon, which served as the substrate; and a middle layer with a carbon nanotube (CNT) electrode array that was screen-printed on the nylon fabric using CNT ink. Polyurethane was added to the ink to make this electrode durable during washing. When the other frictional layer of human skin rubs against the silk, it generates charges to power the electrode. Tests showed that this device is capable of generating electrical signals when pressed with a finger. By connecting it to a computer, the researchers were able to use these signals to perform tasks such as opening a TXT file, access chatting software, and wirelessly trigger home appliances. The authors suggest that this washable electronic textile shows potential in multifunctional wearable devices and human−machine interface systems. 5070
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ASSESSING NANOCELLULOSE FRAGMENTATION AND CRYSTALLIZATION Nanocellulose materials have a variety of useful properties that give them promise for high strength, energy storage, and insulation applications. To bring these applications to fruition, it is critical to understand nanostructural and nanomechanical details of these materials, such as how they kink and break. Although it is well-known that mechanical energy and acid hydrolysis breaks cellulose nanofibrils into shorter particles, where and how this fragmentation occurs remains largely unknown. Specifically, it is unclear whether fibril breakage appears at kinks or along rigid segments, if kinks can be introduced mechanically, and if acid hydrolysis occurs preferentially at positions of kinks. In addition, it is unclear how these fragments assemble into cholesteric liquid crystals. To answer these questions, Nyström et al. (DOI: 10.1021/ acsnano.8b00512) investigated cellulose nanofibril (CNF) fragmentation and crystallization through a combined microscopic and statistical analysis. Their results show that after sonification, the initial mechanical breakage occurs along a segment and not at the position of a kink. Sonification also introduced new kinks. After treating CNFs with hydrochloric acid, the researchers found that breakage events occurred preferentially at kinks but also on random positions in segments between kinks. Direct microscopic observations showed that right-handed carboxylated cellulose nanocrystals self-organize into cholesteric tactoids and films that are left-handed, indicating an inversion of chirality between single particles and liquid crystals. The authors suggest that these findings could offer a critical step forward in applying these materials in nanotechnologies.
strength of the topological charge. By amassing detailed information from the direct visualization of each trajectory, gathered by dark-field digital microscopy, the researchers discovered a two-step mechanism similar to a biomolecular exchange reaction or the Michaelis−Menten scheme. Further examination showed that the total event rate increased with driving force, a result of an increase in the rate of encounters. The driving force did not affect the rate of barrier crossing because this process depended on a random, thermally activated fluctuation of one particle, which enabled the other to pass. The authors suggest that this approach could be used to study a variety of other chemical and physical processes by tailoring specific forces and interactions to reflect the behavior in an analogous system or to examine idealized theoretical scenarios.
GROWING WHERE CRYSTALLINE SEMICONDUCTORS HAVE NEVER GROWN BEFORE Epitaxial growth techniques have thus far been essential for manufacturing nearly all non-silicon integrated electronic and photonic devices. This limitation has prevented the growth of high-quality crystalline materials on amorphous and nonepitaxial substrates, restricting the development of new highperformance materials. Although researchers have used workarounds to try to bypass these restrictions, including nanomaterial growth and device fabrication and epitaxial transfer, these techniques have drawbacks, such as the production of off-target materials and cost and scalability challenges. Seeking a way to grow crystalline materials directly on amorphous and non-epitaxial substrates, Sarkar et al. (DOI: 10.1021/acsnano.8b01819) developed a generalized templated liquid-phase growth technique that enables crystal growth on any substrate, regardless of epitaxial relation. In this technique, a metal with a capping oxide layer is deposited in a desired templated pattern on a substrate, which is then heated in H2, causing the metal to melt. At the growth temperature, a precursor gas for the crystal is introduced, causing the target material to precipitate into a single crystalline material on the template. Using this method, the team was able to grow InP GaP, InAs, InGaP, SnP, and Sn4P3 directly on SiO2, Si3N4, TiO2, Al2O3, Gd2O3, SrTiO3, and graphene. Various tests showed excellent quality of the grown material. Notably, the researchers were able to demonstrate an atomically sharp lateral heterojunction between InP and Sn4P3, materials with vastly different crystal structures. The authors suggest that this approach could impact a wide range of fields, including electronics, photonics, and energy devices.
CROSSING A BARRIER FOR DIRECT PARTICLE VISUALIZATION Many models for microscopic chemical and physical processes have been developed using kinetics inferred from ensemble measurements. However, these statistical interpretations do not account for individual particle motion and deviations from an averaged mechanism. Several recent techniques enable these processes to be studied on an individual event or molecule basis, with repetitions of identical experiments generating data that replace ensemble averages with probability distributions. Figliozzi et al. (DOI: 10.1021/acsnano.8b02012) used this idea to study barrier-crossing dynamics of pairs of nanoparticles in an optical ring trap. The researchers confined pairs of Ag nanoparticles and an azimuthal phase gradient caused the nanoparticles to be driven around the trap in a quasi-onedimensional path. This driving force was controlled by the 5071
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NOT A DROP IN THE BUCKET FOR PITCHER PLANTS Liquid impact on solid surfaces is familiar in nature, such as raindrops bouncing off of leaves, and it is also central to many technical applications, such as inkjet printing, painting, coating, self-cleaning, and crop spraying. Because of interest in developing nonwetting, self-cleaning, and anti-icing surfaces, the majority of research in this area has focused on superhydrophobic substrates. In contrast, comparably fewer studies have focused on dynamic wetting on superhydrophilic substrates, particularly the role of surface curvatures influencing the physics of the drop collision process. Yu et al. (DOI: 10.1021/acsnano.8b01800) studied this phenomenon in the tropical carnivorous pitcher plant, Nepenthes alata. Imaging with a scanning electron microscope (SEM) shows that the surface of the peristome is covered by a two-order hierarchy of parallel microgrooves. Higher-magnification reveals overlapping, air-trapping microcavities that form smaller microgrooves. Together, these features lead to interesting spreading dynamics when a drop contacts this hydrophilic surface: water first spreads in the axial direction, then after 2.6 ms, water begins to spread in the azimuthal direction. To investigate this phenomenon, the researchers used three-dimensional printing to fabricate peristome-mimetic surfaces with varying curvatures. Evidence collected from a high-speed camera suggests that the surface curvature works synergistically with forces including the impact dynamic pressure, the air pressure, the capillary force, and the friction resistance, along with the drop’s impact velocity. The authors suggest that these findings could be used to help design surfaces for applications in food processing, moisture transfer, heat management, inkjet printing, and a variety of other agricultural and industrial processes.
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